Non-aqueous electrolyte secondary battery and battery pack using the same

ABSTRACT

A non-aqueous electrolyte secondary battery includes an electrode assembly formed by winding positive and negative electrodes, and an insulating layer together. Each of the electrodes has a core sheet and mixture layers formed on both sides of the sheet. The insulating layer electrically insulates the electrodes. At least one of the electrodes includes a core-exposed portion continuous parallel to the winding direction. Each of the mixture layers has an inclined weight region where the amount of mixture per unit area decreases toward the core-exposed portion, and a constant weight region in which the amount of mixture per unit area is constant. The inclined weight region has a width of not more than 0.2 of the width of the mixture layers and has an average mixture density of not less than 40% and not more than 99% of the mixture density of the constant weight region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-aqueous electrolyte secondarybattery for high output use, and more particularly, to an inexpensiveand safe non-aqueous electrolyte secondary battery and a battery packusing the battery.

2. Background Art

Non-aqueous electrolyte secondary batteries such as lithium ionsecondary batteries have a higher energy density than other storagebatteries. Because of this advantage, non-aqueous electrolyte secondarybatteries are expanding their market from consumer use such as portableappliances to power tool use such as electric tools.

In hybrid electric vehicle use, for example, if only a large amount ofcurrent can be quickly taken out of a non-aqueous electrolyte secondarybattery when the vehicle is started or accelerated, the vehicle can bedriven afterwards by the internal combustion engine. On the other hand,in electric tool use where the non-aqueous electrolyte secondary batteryis the only power source, a large load is required to start the motor.Particularly when the motor is started from the condition in which thetool is in contact with the operating object, the battery is required toproduce a larger output.

These operating circumstances require high-output secondary batteries toimprove their output characteristics, for which a reduction in internalresistance is inevitable. The reduction in internal resistance isgreatly affected by the current collecting structure in the electrodes(the positive electrode and the negative electrode). More specifically,the internal resistance can be reduced by attaching a current collectorcollectively to the wound parts of an exposed portion having no mixtureapplied thereon of each core sheet of the electrodes. This technique isalready in practical use in nickel-cadmium batteries and nickel-hydrogenbatteries in electric tool and hybrid electric vehicle uses. In thesebatteries, the core sheets are at least as large as 300 μm in thickness,making it possible to attach a current corrector collectively to thewound parts of an exposed portion at an end of each electrode.

In non-aqueous electrolyte secondary batteries, on the other hand, thecore sheets only have a thickness of several tens of micrometers.Therefore, the core sheets need to have a core-exposed portion at theirends so that a current collector can be attached collectively to thewound parts of the core-exposed portion. As methods for forming thecore-exposed portion, there have been various suggestions. For example,Japanese Patent Unexamined Publication No. H10-144301 suggests removingpart of the mixture layers formed on each electrode. Japanese PatentUnexamined Publication No. H11-354110 suggests protecting an area thatis to be the core-exposed portion with a tape and then removing the tapeafter the area is coated with mixture layers.

In these methods, however, the process of removing the mixture layersdegrades the productivity, and the use of the masking material, which isan expendable supply, not only requires the applying and removingprocesses, but also boosts the cost.

Japanese Patent Unexamined Publication No. 2003-20890, on the otherhand, suggests forming the mixture layers excluding the end portions ofthe electrodes so as to make the core-exposed portions at the endportions. In this case, the mixture is applied in such a manner as toswell on the boundary between the core-exposed portion and the mixturelayer, and the thickness of the mixture layer is smoothed in a laterrolling process.

The formation of the electrodes in this manner causes a problemaccording to safety. More specifically, the application of the mixturein such a manner as to swell on the boundary between the core-exposedportion and the mixture layer of the positive electrode increases theweight of the positive electrode on the boundary. This causes thenegative electrode to have a load exceeding the load design value at theportion opposing the boundary on the positive electrode. As a result,lithium ions that cannot be stored in the negative electrode may depositas metallic lithium on the surface of the negative electrode. It isknown that an increase in the amount of lithium ions to be stored in thenegative electrode leads to a decrease in the thermal stability of thenegative electrode. That is why the load design of the negativeelectrode is very important. Especially, high-output secondary batteriesare required to be large in size for the purpose of improving outputcharacteristics. The increased output and size causes an increase in theinternal energy and a decrease in the thermal stability of thenon-aqueous electrolyte secondary batteries. Therefore, the electrodedesign is very important to the batteries.

SUMMARY OF THE INVENTION

The present invention aims to provide a safe and productive non-aqueouselectrolyte secondary battery having excellent discharge characteristicas a power supply for high output use, and a battery pack using thebattery.

The non-aqueous electrolyte secondary battery of the present inventionincludes an electrode assembly formed by winding a positive electrode, anegative electrode, and insulating layers together. The positive andnegative electrodes each have a core sheet and mixture layers formed onboth sides of the core sheet. The insulating layers electricallyinsulate the positive and negative electrodes. At least one of thepositive and negative electrodes includes a core-exposed portioncontinuous parallel to the winding direction. The mixture layers eachhave an inclined weight region and a constant weight region. In theinclined weight region, the amount of mixture per unit area decreases inparallel with and toward the core-exposed portion. In the constantweight region which is adjacent to the inclined weight region, theamount of mixture per unit area is constant. The inclined weight regionhas a width of not more than 0.2 of the width of the mixture layers andhas an average mixture density of not less than 40% and not more than99% of the mixture density of the constant weight region.

As described above, when the core-exposed portion is formed at least oneof the positive and negative electrodes, the weight of the mixture isinclined in the inclined weight region of the mixture layer. Thisstructure can eliminate the process of removing the mixture layers andreducing the number of expendable supplies, thereby enabling the batteryto be manufactured at low cost. The battery also has high safety byreducing the weight of the positive electrode on the end of themixture-coated surface so as to reduce the design load of the opposingnegative electrode. The battery configuration of the present inventionmakes the non-aqueous electrolyte secondary battery highly productiveand safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a developed schematic view showing a positive electrode, anegative electrode, and a separator of a non-aqueous electrolytesecondary battery in accordance with an embodiment of the presentinvention.

FIG. 1B is a sectional view of an essential part of the non-aqueouselectrolyte secondary battery in accordance with the embodiment of thepresent invention.

FIG. 2A is a schematic view of the non-aqueous electrolyte secondarybattery in accordance with the embodiment of the present invention.

FIG. 2B is a schematic view of another non-aqueous electrolyte secondarybattery in accordance with the embodiment of the present invention.

FIG. 3 is a schematic view of a battery pack using the non-aqueouselectrolyte secondary battery in accordance with the embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a developed schematic view showing a positive electrode, anegative electrode, and a separator of a non-aqueous electrolytesecondary battery in accordance with an embodiment of the presentinvention. FIG. 1B is a sectional view of an essential part of thenon-aqueous electrolyte secondary battery in accordance with theembodiment of the present invention. FIG. 2A is a schematic view of thenon-aqueous electrolyte secondary battery in accordance with theembodiment of the present invention.

Electrode assembly 5 shown in FIG. 2A is formed by winding positiveelectrode 1 and negative electrode 3 shown in FIG. 1A together withseparator 4 therebetween. As shown in FIG. 1B, positive electrode 1includes positive electrode core sheet 22 and positive-electrode mixturelayers 24 formed on both sides of positive electrode core sheet 22.Positive-electrode mixture layers 24 each have insulating layer 31 ontheir surface. Negative electrode 3 includes negative electrode coresheet 23 and negative-electrode mixture layers 25 formed on both sidesof negative electrode core sheet 23. Positive and negative electrodes 1and 3 include core-exposed portions 2C and 2A, respectively, which arecontinuous parallel to the winding direction.

As shown in FIG. 2A, after electrode assembly 5 is complete, currentcollector 6C is attached to core-exposed portion 2C, and currentcollector 6A is attached to core-exposed portion 2A. In this case,current collector 6C is collectively welded to the wound parts ofcore-exposed portion 2C, and current collector 6A is collectively weldedto the wound parts of core-exposed portion 2A. Later, electrode assembly5 is housed in battery can 7; current collector 6A is connected tobattery can 7; and current collector 6C is connected to an unillustratedlid. Finally, battery can 7 is filled with an unillustrated non-aqueouselectrolytic solution and closed by being crimped together with the lidso as to complete a non-aqueous electrolyte secondary battery.

The core-exposed portion may be formed only on positive electrode 1 andnot on negative electrode 3 as shown in FIG. 2B. In this case,negative-electrode mixture layers 25 are partly removed in the directionperpendicular to the winding direction, and the removed portion iswelded with current collector 61A. Although not illustrated, it is alsopossible to form the core-exposed portion only on negative electrode 3and not on positive electrode 1.

The positive active material to be contained in positive-electrodemixture layers 24 can be any known positive electrode materialcontaining a sufficient amount of lithium ions and capable of storingand emitting lithium ions. Desirable examples of the positive activematerial include compound metal oxides composed of lithium and atransition metal which are expressed by a general formula: LiM_(x)O_(y)and lithium-containing intercalation compounds. In the general formula,1<x≦2; 2<y≦4; and M contains at least one of cobalt (Co), nickel (Ni),manganese (Mn), iron (Fe), aluminum (Al), vanadium (V), and titanium(Ti).

The binder to be contained in positive-electrode mixture layers 24 maybe any known binder commonly used in the positive-electrode mixturelayers of batteries of this kind. Specific examples of the binderinclude: polyethylene, polypropylene, polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), and styrene-butadiene rubber.Positive-electrode mixture layers 24 may be added with a known additiveagent or other agents when necessary. More specifically, a conductiveagent such as carbon black may be added.

Positive electrode core sheet 22 and current collector 6C can be made ofaluminum (Al), titanium, or the like, and either positive electrode coresheet 22 or current collector 6C may be used after being surface-treatedwith carbon or the like. The carbon surface treatment is appliedexcluding core-exposed portion 2C.

The negative active material to be contained in negative-electrodemixture layers 25 can be a carbon material, a crystalline or amorphousmetal compound, or the like which are capable of storing and emittinglithium ions. Specific examples of the carbon material includenon-graphitizable carbon materials such as cokes and glassy carbons,highly crystalline carbon materials with a well-developed crystalstructure, such as graphite materials. More specific examples of thecarbon material include pyrolytic carbons, cokes (pitch cokes, needlecokes, petroleum cokes, and the like), graphites, glassy carbons, thefired materials of organic polymer compounds (materials obtained bycarbonizing phenolic resin, furan resin, or the like by being fired atappropriate temperatures), carbon fibers, and activated carbons.

The binder to be contained in negative-electrode mixture layers 25 canbe polyethylene, polypropylene, PTFE, PVDF, or styrene-butadiene rubber.The binder may be any known binder commonly used in thenegative-electrode mixture layers of batteries of this kind.Negative-electrode mixture layers 25 may be added with a known additiveagent or other agents when necessary.

Negative electrode core sheet 23 and current collector 6A can be made ofstainless steel, nickel, copper, titanium, or the like. Either negativeelectrode core sheet 23 or current collector 6A may be used after beingsurface-treated with carbon, nickel, titanium, or the like. The carbonsurface treatment is applied excluding core-exposed portion 2A.

The non-aqueous electrolytic solution is prepared by dissolving anelectrolyte (supporting electrolyte) in a non-aqueous solvent. Thenon-aqueous solvent contains, as the main solvent, ethylene carbonate(hereinafter, EC), which has a comparatively high dielectric constantand is hard to be degraded by the graphite contained in negativeelectrode 3. The use of EC as the main solvent is particularly desirablewhen the negative active material contains a graphite material. However,it is alternatively possible to use a compound in which hydrogen atomsof EC are replaced by halogen.

Part of EC or the compound in which the hydrogen atoms of EC arereplaced by halogen as the main solvent can be replaced by a secondcomponent solvent to obtain better properties. The second componentsolvent can be something that reacts with a graphite material such aspropylene carbonate (hereinafter, PC). Specific examples of the secondcomponent solvent other than PC include: butylene carbonate, vinylenecarbonate, 1,2-dimethoxyethane, 1,2-dimethoxymethane, γ-butyrolactone,valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane,4-methyl-1,3-dioxolane, sulfolane, and methylsulfolane.

The non-aqueous solvent is preferably used with a low viscosity solventto increase conductivity, thereby improving current characteristics andalso to reduce reactivity with metallic lithium, thereby improvingsafety. Specific examples of the low viscosity solvent include:symmetric or asymmetric chain carbonic esters such as diethyl carbonate,dimethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate;symmetric or asymmetric chain carbonic carboxylic esters such as methylpropionate and ethyl propionate; and phosphate esters such as trimethylphosphate and triethyl phosphate. These low viscosity solvents can beused either on their own or in combination.

The electrolyte can be any lithium salt that is dissolved in anon-aqueous solvent and is ion conductive. Specific examples of theelectrolyte include: LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiB(C₆H₅)₄, LiCH₃SO₃,CF₃SO₃Li, LiCl, LiBr, and the like. Of these, LiPF₆ is most desirable.These electrolytes can be used either on their own or in combination.

The electrolyte of the non-aqueous electrolyte battery is not limited tothe aforementioned non-aqueous electrolytic solution and can be a solidor gel electrolyte. The non-aqueous electrolyte battery, which iscylindrical in the present embodiment, can be formed in other shapessuch as a rectangular column or a plate and can be thin, large, or anyother size.

Separator 4 can be a microporous membrane made of polyolefin such aspolyethylene or polypropylene.

Battery can 7 can be made of iron, nickel, stainless steel, aluminum,titanium, or the like and may be subjected to plating or othertreatments to avoid electrochemical corrosion caused by the non-aqueouselectrolytic solution during charge-discharge cycles.

Insulating layers 31 are porous layers which are made of an insulatingmaterial and can permeate a non-aqueous electrolytic solution. In otherwords, insulating layers 31 are permeable to lithium ion. The inorganicfiller to be contained in insulating layers 31 is made of aluminum oxide(alumina), silicon dioxide, titanium dioxide, zirconium oxide, magnesiumoxide, or resin all of which being in powder form. These materials canbe used either on their own or in combination. The inorganic filler isnot limited in shape.

Insulating layers 31 prevent a short circuit between positive electrode1 and negative electrode 3 like separator 4, and therefore separator 4is not necessary in some battery configurations. Insulating layers 31formed on positive-electrode mixture layers 24 in the present embodimentmay be alternatively formed on negative-electrode mixture layers 25, onboth layers 24 and 25, or on separator 4.

However, it is still desirable to provide separator 4 because when thebattery reaches an abnormally high temperature, separator 4 is melted toclose its pores so as to block the flow of current, thereby improvingthe safety of the battery. In order to weld current collectors 6A and 6Cto core-exposed portions 2A and 2C, respectively, both ends of separator4 are required to be at least inside the ends of core-exposed portions2A and 2C.

The following is a description of weight distribution onpositive-electrode mixture layers 24 and on negative-electrode mixturelayers 25 in the present embodiment. Positive-electrode mixture layers24 will be described first.

Each of positive-electrode mixture layers 24 has constant weight region51 and inclined weight region 52. Inclined weight region 52 is formed tobe adjacent to and substantially parallel to core-exposed portion 2C.Inclined weight region 52 has a less amount of mixture per unit area asit is closer to core-exposed portion 2C. In other words, in inclinedweight region 52, the amount of mixture per unit area in parallel withcore-exposed portion 2C decreases toward core-exposed portion 2C.

Constant weight region 51 is adjacent to inclined weight region 52 andis substantially constant in the amount of mixture per unit area.Assuming that constant weight region 51 has a width “A”, and inclinedweight region 52 has a width “B”, the relation “0<B/(A+B)≦0.2” issatisfied.

Inclined weight region 52 has an average mixture density of not lessthan 40% and not more than 99% of the mixture density of constant weightregion 51, and the average mixture density is controlled byroll-pressing positive-electrode mixture layers 24. When the width “B”of inclined weight region 52 exceeds 0.2 of the width (A+B) ofpositive-electrode mixture layer 24, the battery capacity decreases. Onthe other hand, when the ratio of the average mixture density ofinclined weight region 52 to the mixture density of constant weightregion 51 is less than 40%, positive electrode core sheet 22 andpositive-electrode mixture layers 24 have an insufficient bondingstrength therebetween after the roll-pressing of positive-electrodemixture layers 24. As a result, the mixture is highly likely to fall offpositive electrode core sheet 22 if subjected to an impact or vibration.To avoid this, inclined weight region 52 has an average mixture densityof not less than 40% and not more than 99% of the mixture density ofconstant weight region 51.

Similarly, each of negative-electrode mixture layers 25 has constantweight region 53 and inclined weight region 54. Assuming that constantweight region 53 has a width “A” and inclined weight region 54 has awidth “B”, the relation “0<B/(A+B)≦0.2” is satisfied. Inclined weightregion 54 has an average mixture density of not less than 40% and notmore than 99% of the mixture density of constant weight region 53. Theconstant weight region and the inclined weight region, which are formedin both positive-electrode mixture layers 24 and negative-electrodemixture layers 25 in the present embodiment, may be formed in eitherlayers 24 or layers 25.

Insulating layers 31 preferably contain a heat-resistant material.Separator 4, which is a microporous membrane made of a polymer resin,has the property of shrinking at a high temperature. In vehicle andother uses with a severe use environment, the shrinkage of separator 4in addition to the heat generated during discharge may cause a shortcircuit between positive electrode 1 and negative electrode 3, and mayeven cause heat or smoke. However, the use of a heat-resistant materialin insulating layers 31 is desirable in terms of safety because it cankeep the insulation between positive electrode 1 and negative electrode3 when separator 4 is shrunk. It is also possible to addlow-melting-point resin beads to insulating layers 31 in order toprovide the function of blocking the current at an abnormally hightemperature. Alternatively, the resin beads may be formed into a layerand provided on each insulating layer 31. Insulating layers 31 can beformed by stirring a precursor solution containing a heat-resistantmaterial in a double-arm kneader until the solution is kneaded into apaste, and then by applying the paste to positive electrode 1, negativeelectrode 3 or separator 4 by doctor blading or die coating and dryingit.

The heat-resistant material used in insulating layers 31 is preferably aheat-resistant resin having a heat deflection temperature of 200° C. orhigher. Specific examples of the heat-resistant material includepolyimide, polyamide-imide, aramid, polyphenylene sulfide,polyetherimide, polyethylene terephthalate, polyethernitrile,polyetherketone, and polybenzimidazole. Of these, an aramid resin ismost desirable because of its high heat deflection temperature.

Insulating layers 31 desirably also contain an insulating filler toincrease the porosity, thereby improving the capacity to retain theelectrolytic solution. This can also improve battery characteristics. Itis particularly desirable to use as the main material an insulatingfiller whose particles are bonded to each other via a binder. Specificexamples of the binder to bond the particles of the insulating fillerinclude PTFE and modified acrylonitrile rubber particles, other thanPVDF. In the case of using PTFE or modified acrylonitrile rubberparticles, it is desirably combined with a viscosity improver such ascarboxymethylcellulose (hereinafter, CMC), polyethylene oxide (PEO), ormodified acrylonitrile rubber that has a viscosity different from themodified acrylonitrile rubber particles used as the binder. These resinshave a high affinity for a non-aqueous electrolytic solution so thatthey absorb the solution and swell. This swelling allows insulatinglayers 31 to properly expand their volume, thereby improving thecapacity to retain the non-aqueous electrolytic solution. As theinsulating filler, resin beads, an inorganic oxide, or the like can beused and an inorganic oxide is desirable because of its high specificheat. Above all, alumina, titania, zirconia, and magnesia areparticularly desirable because of their high specific heat, thermalconductivity, and thermal shock resistance. Although the insulatingfiller and the heat resistant resin can be used separately because oftheir heat resistance, they can alternatively be mixed or layered on topof each other.

Insulating layers 31 containing a heat-resistant material may beprovided in both positive electrode 1 and negative electrode 3; however,it is preferably provided in either electrode 1 or 3 in order to reducethe number of production steps. In this case, insulating layers 31 arepreferably provided on negative electrode 3 so as to improve theinsulation between positive electrode 1 and negative electrode 3 becausenegative electrode 3 is larger in area than positive electrode 1.

One of insulating layers 31 containing a heat-resistant material isdesirably supported on separator 4 because it can prevent separator 4from being shrunk due to the heat generated during the welding of thecurrent collectors to the electrode core sheets.

Using the non-aqueous electrolyte secondary battery thus structured, abattery pack described as follows with reference to FIG. 3 is formed.FIG. 3 is a perspective view of the battery pack. Five non-aqueouselectrolyte secondary batteries (hereinafter, batteries) 41 are arrangedin parallel at intervals of, for example, 1 mm using unillustratedisolating plates. Five batteries 41 are connected in series with eachother by resistance-welding linkage plates 8 to the positive terminal ofone of two adjacent batteries 41 and to the negative terminal of theother of the two adjacent batteries 41. The positive terminal of anoutermost battery 41B is connected to the positive terminal 10, and thenegative terminal of the other outermost battery 41C is connected tonegative terminal 11. Linkage plates 8 can be made of Fe, Ni, Al, Ti,stainless steel, copper (Cu) or the like, and are desirably made of Alor Cu because of their low electrical resistance.

Battery 41A arranged in the center is equipped with temperature sensor 9in order to measure the temperature during charge-discharge cycles.Temperature sensor 9 is formed of a thermocouple or the like and isattached on an insulating tube of battery 41A so as to be connected toan unillustrated controller. The controller may be designed to open thecircuit between positive terminal 10 and negative terminal 11 when theoutput of temperature sensor 9 reaches a predetermined value (60° C.,for example). This prevents the battery pack from increasing intemperature.

Five batteries 41, linkage plates 8, and temperature sensor 9 arecovered with outer case 12 made of resin. Outer case 12 is made of amaterial having high heat resistance and high mechanical strength suchas acrylonitrile-styrene-butadiene (ABS), polyethylene terephthalate(PET), and polypropylene (PP).

The following are specific examples of batteries to describe advantagesof the present invention. Preparation of main components of Battery no.1 as a first sample will be described first. In addition, the width “B”of inclined weight region 52 relative to the width (A+B) ofpositive-electrode mixture layer 24 will be determined as follows.

(1) Production of the Positive Electrode

As a positive active material, Li_(1.0)Ni_(1/3)Mn_(1/3)Co_(1/3)O₂ isused. To obtain this material, lithium carbonate (Li₂CO₃) is mixed, in aspecific number of moles, with hydroxide consisting of nickel,manganese, and cobalt (an equimolar mixture of Ni(OH)₂, Mn(OH)₂, andCo(OH)₂) and then fired for 10 hours at 900° C. in air atmosphere.

The hydroxide consisting of nickel, manganese, and cobalt is synthesizedby dissolving a predetermined amount of each of nickel nitrate,manganese nitrate, and cobalt nitrate together; adding sodium hydroxidethereto; washing the coprecipitate; and drying it at 150° C.

This positive active material is agitated to be mixed with acetyleneblack as a conductive material and N-methylpyrrolidone solution ofpolyvinylidene fluoride as a binder so as to obtain positive-electrodemixture paste. The positive active material, acetylene black, andpolyvinylidene fluoride are mixed in a weight ratio of 100:3:5. Then,the positive-electrode mixture paste is applied to both surfaces of a 15μm-thick aluminum foil which is used as positive electrode core sheet22. An end portion of positive electrode core sheet 22 is formed as anon-coated portion having a width of 20 mm. This non-coated portioncorresponds to core-exposed portion 2C. In Battery no. 1, thepositive-electrode mixture paste is applied in such a manner that thewidth “B” of inclined weight region 52 can satisfy “B/(A+B)=0.1”. Thewidth “B” is determined by measuring the difference in the thickness ofthe applied and dried paste. The total width (A+B) to be coated is setto 50 mm and the width “B” is set to 5 mm.

The inclination of weight is achieved by adjusting the viscosity andcoating speed of the paste. More specifically, the viscosity of thepaste is adjusted to be in the range of 13000 to 16000 cPs at roomtemperature using a Brookfield viscometer, and the paste is coated 5 mto 15 m per minute. Later, the coated paste is dried and rolled by arolling mill, and positive electrode core sheet 22 with the paste is cutin size so as to obtain positive electrode 1.

Then, the average mixture density on the end of the coated mixture,which is inclined weight region 52, is calculated from the thickness andweight of the mixture applied. The mixture density of constant weightregion 51 having no inclination of weight is calculated in the samemanner. In Battery no. 1, the mixture density ratio, which is the ratioof the average mixture density of inclined weight region 52 to themixture density of constant weight region 51, is set at 70%.

(2) Production of the Negative Electrode

As a negative active material, scaly graphite is used which ispulverized and classified to have an average particle diameter of about20 μm. The scaly graphite is mixed with styrene-butadiene rubber as thebinder in a weight ratio of 100:3. The graphite mixed with the binder isadded with a CMC aqueous solution in such a manner that CMC is 1% of thegraphite, thereby obtaining negative-electrode mixture paste. Thenegative-electrode mixture paste is applied to both surfaces of a 10μm-thick copper foil which is used as negative electrode core sheet 23.After dried, the paste is roll-pressed by a rolling mill, and negativeelectrode core sheet 23 with the paste is cut in a predetermined size.The negative electrode mixture is partly removed in the directionperpendicular to the winding direction, and the removed portion isultrasonic welded with nickel current collector 61A shown in FIG. 2. Asa result, the negative electrode is complete.

(3) Preparation of the Non-Aqueous Electrolytic Solution

As a solvent, a mixture of EC and ethyl methyl carbonate is used in avolume ratio of 30:70 at 40° C. LiPF₆ is dissolved at 1.0 mol/L in thissolvent. This is the completion of the non-aqueous electrolyticsolution.

With the aforementioned components, the non-aqueous electrolytesecondary battery is prepared as follows. Positive electrode 1 andnegative electrode 3 are wound together via polyethylene separator 4therebetween to form electrode assembly 5 shown in FIG. 2B. The end ofthe negative-electrode mixture layer is provided to protrude as long as2 mm from the end of the positive-electrode mixture layer. Later,current collector 6C is collectively welded to the wound parts ofcore-exposed portion 2C formed in positive electrode 1. Then, electrodeassembly 5 is housed in battery can 7; current collector 6C is connectedto an unillustrated lid; and current collector 61A is connected tobattery can 7. Finally, battery can 7 is filled with the non-aqueouselectrolytic solution and sealed with the lid, thereby completing thenon-aqueous electrolyte secondary battery as Battery no. 1. Battery no.1 is 18 mm in diameter and 65 mm in height.

Battery no. 2 is prepared in the same manner as Battery no. 1 exceptthat while the width (A+B) of positive-electrode mixture layer 24 is 50mm, the width “B” of inclined weight region 52 is set to 10 mm. Morespecifically, the positive-electrode mixture paste is applied so as tosatisfy “B/(A+B)=0.2” by adjusting the viscosity of the paste to therange of 9000 to 12000 cPs at room temperature. In the followingdescription, Battery no. 2 will be used as a typical example.

Battery no. 3 is prepared in the same manner as Battery no. 1 exceptthat while the width (A+B) of positive-electrode mixture layer 24 is 50mm, the width “B” of inclined weight region 52 is set to 15 mm. Morespecifically, the positive-electrode mixture paste is applied so as tosatisfy “B/(A+B)=0.3” by adjusting the viscosity of the paste to therange of 5000 to 8000 cPs at room temperature. Specifications ofBatteries no. 1 to 3 are shown in Table 1 as below.

TABLE 1 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 1 0.1 70 nocore-exposed polyethylene portion 2 0.2 70 no core-exposed polyethyleneportion 3 0.3 70 no core-exposed Polyethylene portion

Using five cells of each of these non-aqueous electrolyte secondarybatteries, non-aqueous electrolyte secondary battery packs as shown inFIG. 3 are produced and evaluated by the following tests.

Measurement of Discharge Capacity

Each battery pack is charged and discharged at 25° C. Charging isperformed at a constant current of 2 A until each battery reaches 4.2Vand then at a constant voltage of 4.2V. The charging is completed whenthe charge current goes down to 200 mA. On the other hand, dischargingis performed at a constant current of 10 A until each battery reaches2.5V. The charging and discharging have a rest period of 20 minutesbetween them. The charging and discharging operations excluding thecurrent values and the rest period are controlled by a controller whichis installed in the battery pack and has a function of equalizing thestate of charge between the batteries in the battery pack.

Observation of Lithium Deposition After Low-Temperature Cycle Test

In order to examine whether or not the accepting load of lithium ion bythe negative electrode exceeds the load design value depending on thedifference in density on the end portion of positive electrode 1, eachbattery pack is charged and discharged for 200 cycles at 0° C. Chargingis performed at a constant current of 2 A until each battery reaches4.25V and then at a constant voltage of 4.25V. The charging is completedwhen the charge current goes down to 200 mA. On the other hand,discharging is performed at a constant current of 10 A until eachbattery reaches 2.5V. The charging and discharging have a rest period of20 minutes between them. After 200 cycles, battery 41A in the center ofthe battery pack shown in FIG. 3 is decomposed while it is in a chargedstate to examine the deposition of metallic lithium in the portion ofthe negative electrode that is opposed to the end portion of positiveelectrode 1.

Vibration Test

Each battery pack is subjected to a 10-hour vibration test using a pulsewidth of 50 Hz at 20 G. The difference in open circuit voltage (OCV)between before and after the test is measured.

Storage Test

Each battery pack is charged and discharged at 25° C. at first. Chargingis performed at a constant current of 2 A until each battery reaches4.2V and then at a constant voltage of 4.2V. The charging is completedwhen the charge current goes down to 200 mA. On the other hand,discharging is performed at a constant current of 10 A until eachbattery reaches 2.5V. The charging and discharging have a rest period of20 minutes between them. After the discharging, battery packs are storedfor three days at 80° C. Other battery packs are stored for six hours at100° C. After the respective storage periods, each one cell in thebattery packs is decomposed to determine the maximum shrinkage of theseparator.

Results of these tests are shown in Table 2 as below.

TABLE 2 OCV Lithium decrease Shrinkage Shrinkage Discharge deposition inafter 3 after 6 Buttery capacity in cycle vibration days at hours at no.(Ah) test test (V) 80° C. (%) 100° C. (%) 1 1.61 no 0.000 0 4.5 2 1.48no 0.001 0 4.5 3 1.23 no 0.001 0 4.6

Table 2 shows that when the value of B/(A+B), which is the ratio of thewidth of inclined weight region 52 to the width of positive-electrodemixture layer 24, exceeds 0.2, increase of the width “B” causesreduction in the weight of positive-electrode mixture layer 24, therebygreatly decreasing the discharge capacity. However, Batteries no. 1 to 3have similar estimation results in the other aspects, and therefore thevalue of B/(A+B) is desirably not more than 0.2.

The following is a description of the ratio of the average mixturedensity of inclined weight region 52 to the mixture density of constantweight region 51 after the mixture is roll-pressed.

Battery no. 4 is prepared in the same manner as Battery no. 1 exceptthat positive-electrode mixture layers 24 are provided at their endportions at the same side with an organic-solvent-resistant foamedmaterial so as to increase the thickness of the mixture to be applied tothe end portions. More specifically, inclined weight region 52 is notprovided, and instead the portion corresponding to inclined weightregion 52 is designed to have a mixture density of 105% of the mixturedensity of constant weight region 51 after the mixture is roll-pressed.

Batteries no. 5, 6, and 7 are prepared in the same manner as Battery no.2 except that the mixture is roll-pressed with different roller gapsbetween constant weight region 51 and inclined weight region 52. As aresult, the ratio of the average mixture density of inclined weightregion 52 to the mixture density of constant weight region 51 after theroll-pressing of the mixture is 99%, 40%, and 30% in Batteries no. 5, 6,and 7, respectively.

Specifications of Batteries no. 2 and 4 to 7 are shown in Table 3.

TABLE 3 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 4 0 105 no core-exposedpolyethylene portion 5 0.2 99 no core-exposed polyethylene portion 2 0.270 no core-exposed polyethylene portion 6 0.2 40 no core-exposedpolyethylene portion 7 0.2 30 no core-exposed polyethylene portion

Battery packs prepared using Batteries no. 4 to 7 are estimated in thesame manner as Batteries no. 1 to 3 above. The test results of Batteriesno. 4 to 7 are shown in Table 4 as below together with the test resultsof Battery no. 2.

TABLE 4 OCV Lithium decrease Shrinkage Shrinkage Discharge deposition inafter 3 after 6 Battery capacity in cycle vibration days at hours at no.(Ah) test test (V) 80° C. (%) 100° C. (%) 4 1.48 deposited 0.000 0 4.0 51.48 No 0.000 0 4.0 2 1.48 No 0.001 0 4.5 6 1.49 No 0.001 0 4.5 7 1.48no 0.023 0 4.7

In Battery no. 4 in which positive electrode 1 has the increased mixturedensity at its end portions, a larger load is applied to negativeelectrode 3. As shown in Table 4, when Battery no. 4 is decomposed andobserved after being subjected to the test in long-term andlow-temperature environments, it is confirmed that the accepting load oflithium ion by the negative electrode exceeds the load design value.This may cause a short circuit between electrodes 1 and 3, which mayeven cause trouble such as heat or smoke.

On the other hand, when the mixture density ratio is less than 40% as inBattery no. 7, the vibration test shows a decrease in the OCV. Batterieshaving a reduced OCV are decomposed to find that positive-electrodemixture layers 24 are partly released or suspended. The cause of thisseems to be that the decreased mixture density reduces the adhesionbetween positive electrode core sheet 22 and positive-electrode mixturelayers 24. As a result, the mixture density ratio is desirably not lessthan 40% and not more than 99%.

The following is a description of materials of insulating layers 31 wheninsulating layers 31 are provided in positive electrode 1 and separator4 is not provided. Batteries no. 8 to 12 are prepared in the same manneras Battery no. 2 except that positive electrode 1 is provided with 20μm-thick insulating layers 31, which are made of an aramid resin, analumina porous material, a titania porous material, a zirconia porousmaterial, and a magnesia porous material, respectively. Specificationsof Batteries no. 2 and no. 8 to 12 are shown in Table 5 below.

TABLE 5 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 2 0.2 70 nocore-exposed polyethylene portion 8 0.2 70 no core-exposed aramid resinportion 9 0.2 70 no core-exposed alumina portion 10 0.2 70 nocore-exposed titania portion 11 0.2 70 no core-exposed zirconia portion12 0.2 70 no core-exposed magnesia portion

Battery packs prepared using the battery examples are estimated in thesame manner as described above. The test results are shown in Table 6 asbelow together with the test results of Battery no. 2.

TABLE 6 OCV Lithium decrease Shrinkage Shrinkage Discharge deposition inafter 3 after 6 Battery capacity in cycle vibration days at hours at no.(Ah) test test (V) 80° C. (%) 100° C. (%) 2 1.48 no 0.001 0 4.5 8 1.48no 0.000 0 0.2 9 1.48 no 0.000 0 0 10 1.48 no 0.001 0 0 11 1.48 no 0.0010 0 12 1.49 no 0.000 0 0

As shown in Table 6, both separator 4 made of polyethylene andinsulating layers 31 made of a heat-resistant material have excellentresults with similar shrinkage at 80° C. On the other hand, at 100° C.,the batteries using insulating layers 31 have higher insulationreliability. This means that the provision of insulating layers 31 madeof a heat-resistant material can further improve battery safety.

Batteries no. 1 to 12 described hereinbefore have core-exposed portion2C on the positive electrode only. The following is a description ofbattery examples having core-exposed portion 2A on the negativeelectrode only.

(4) Production of the Positive Electrode

The same positive-electrode mixture paste as used in Battery no. 1 isapplied to both surfaces of a 15 μm-thick aluminum foil, which is usedas the positive electrode core. After being dried, the paste isroll-pressed by a rolling mill and positive electrode core sheet withthe paste is cut in size. The positive electrode mixture is partlyremoved in the direction perpendicular to the winding direction, and theremoved portion is ultrasonic welded with a ribbon-like currentcollector made of Al. As a result, the positive electrode 1 is complete.

(5) Production of the Negative Electrode

The same negative-electrode mixture paste as used in Battery no. 1 isapplied to both surfaces of a 10 μm-thick copper foil, which is used asnegative electrode core sheet 23. A non-coated portion having a width of20 mm is formed at the end of negative electrode core sheet 23. Thisnon-coated portion corresponds to core-exposed portion 2A. In Batteryno. 13, the negative-electrode mixture paste is applied in such a mannerthat the width “B” of inclined weight region 54 can satisfy“B/(A+B)=0.1”. The total width (A+B) to be coated is set to 54 mm andthe width “B” is set to 5.4 mm. After dried, the paste is roll-pressedby a rolling mill, and negative electrode core sheet 23 with the pasteis cut in size so as to obtain negative electrode 3. The ratio of themixture density of the non-coated portion, which is inclined weightregion 54 to the mixture density of constant weight region 53 is set at70%.

The non-aqueous electrolyte secondary battery as Battery no. 13 will beprepared as follows. The positive electrode and negative electrode 3thus obtained are wound together with polyethylene separator 4therebetween to form an electrode assembly. The end of thenegative-electrode mixture layer is provided to protrude as long as 2 mmfrom the end of the positive-electrode mixture layer. Later, currentcollector 6A is collectively welded to the wound parts of core-exposedportion 2A formed in negative electrode 3. The electrode assembly ishoused in battery can 7; current collector 6C of the positive electrodeis connected to an unillustrated lid; and current collector 6A isconnected to battery can 7. Finally, battery can 7 is filled with thenon-aqueous electrolytic solution as described previously and sealedwith the lid, thereby completing the non-aqueous electrolyte secondarybattery as Battery no. 13.

Battery no. 14 is prepared in the same manner as Battery no. 13 exceptthat the width (A+B) of negative-electrode mixture layer 25 is set to 54mm and the width “B” is set to 10.8 mm. More specifically, thenegative-electrode mixture paste is applied so as to satisfy“B/(A+B)=0.2”. In the following description, Battery no. 14 will be usedas a typical example.

Battery no. 15 is prepared in the same manner as Battery no. 13 exceptthat while the width (A+B) of negative-electrode mixture layer 25 is 54mm, the width “B” is set to 16.2 mm. More specifically, thenegative-electrode mixture paste is applied so as to satisfy“B/(A+B)=0.3”. Specifications of Batteries no. 13 to 15 are shown inTable 7 as below.

TABLE 7 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 13 no core-exposed 0.170 polyethylene portion 14 no core-exposed 0.2 70 polyethylene portion15 no core-exposed 0.3 70 polyethylene portion

Battery packs prepared using the battery examples are estimated in thesame manner as described above. The test results are shown in Table 8 asbelow.

TABLE 8 OCV Lithium decrease Shrinkage Shrinkage Discharge deposition inafter 3 after 6 Battery capacity in cycle vibration days at hours at no.(Ah) test test (V) 80° C. (%) 100° C. (%) 13 1.48 No 0.000 0 4.5 14 1.48no 0.000 0 4.5 15 1.48 deposited 0.000 0 4.6

In case that the positive active material of a just completed batterycontains lithium ions, the battery capacity is determined by the weightof positive-electrode mixture layers 24. Therefore, as shown in Table 8,a change in the width of inclined weight region 54 of negative electrode3 does not affect the discharge capacity. However, when negativeelectrode 3 has core-exposed portion 2A, if B/(A+B) exceeds 0.2 as inBattery no. 15, negative electrode 3 cannot store all the lithium ionsfrom the opposed positive electrode. This causes the deposition ofmetallic lithium as a result that the amount of the negative electrodemixture to be applied is decreased as the width “B” is increased.Although this phenomenon can be avoided by making positive-electrodemixture layers 24 smaller in width than negative-electrode mixturelayers 25, this results in decrease in the battery capacity. To preventthe decrease, the value of B/(A+B) is desirably not more than 0.2.

The following is a description of the ratio of the average mixturedensity of inclined weight region 54 to the mixture density of constantweight region 53 after the mixture is roll-pressed.

Battery no. 16 is prepared in the same manner as Battery no. 14 exceptthat negative-electrode mixture layers 25 are provided at their endportions at the same side with an organic-solvent-resistant foamedmaterial so as to increase the thickness of the mixture to be applied tothe end portions. More specifically, inclined weight region 54 is notprovided, and instead the portion corresponding to inclined weightregion 54 is designed to have a mixture density of 105% of the mixturedensity of constant weight region 53 after the mixture is roll-pressed.

Batteries no. 17 to 19 are prepared in the same manner as Battery no. 14except that the ratio of the average mixture density of inclined weightregion 54 to the mixture density of constant weight region 53 after themixture is roll-pressed is 99%, 40%, and 30%, respectively.Specifications of Batteries no. 14 and no. 16 to 19 are shown in Table 9as below.

TABLE 9 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 16 no core-exposed 0105 polyethylene portion 17 no core-exposed 0.2 99 polyethylene portion14 no core-exposed 0.2 70 polyethylene portion 18 no core-exposed 0.2 40polyethylene portion 19 no core-exposed 0.2 30 polyethylene portion

Battery packs prepared using the battery examples are estimated in thesame manner as described above. The test results are shown in Table 10as below together with the test results of Battery no. 14.

TABLE 10 OCV Lithium decrease Shrinkage Shrinkage Discharge depositionin after 3 after 6 Battery capacity in cycle vibration days at hours atno. (Ah) test test (V) 80° C. (%) 100° C. (%) 16 1.48 deposited 0.000 04.1 17 1.48 — 0.000 0 4.2 14 1.48 — 0.000 0 4.5 18 1.48 — 0.001 0 4.5 191.48 deposited 0.021 0 4.8

As in Battery no. 16, an increase in the mixture density of the negativeelectrode 3 causes negative electrode 3 to have a lower performance toaccept lithium ions under these experimental conditions. Consequently,deposition of metallic lithium is observed in the region of negativeelectrode 3 where the mixture density is large. On the other hand, inBattery no. 19 having a mixture density ratio as low as 30% as inBattery no. 7, the mixture falls off negative electrode 3 when subjectedto vibration. Therefore, the mixture density ratio is desirably not lessthan 40% and not more than 99%.

The following is a description of materials of insulating layers 31 wheninsulating layers 31 are provided in negative electrode 3, and separator4 is not provided. Batteries no. 20 to 24 are prepared in the samemanner as Battery no. 14 except that negative electrode 3 is providedwith 20 μm-thick insulating layers 31, which are made of an aramidresin, an alumina porous material, a titania porous material, a zirconiamaterial, and a magnesia porous material, respectively. Specificationsof Batteries no. 14 and no. 20 to 24 are shown in Table 11 as below.

TABLE 11 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 14 no core-exposed 0.270 polyethylene portion 20 no core-exposed 0.2 70 aramid resin portion21 no core-exposed 0.2 70 alumina portion 22 no core-exposed 0.2 70titania portion 23 no core-exposed 0.2 70 zirconia portion 24 nocore-exposed 0.2 70 magnesia portion

Battery packs prepared using the battery examples are estimated in thesame manner as described above. The test results are shown in Table 12as below together with the test results of Battery no. 14.

TABLE 12 OCV Lithium decrease Shrinkage Shrinkage Discharge depositionin after 3 after 6 Battery capacity in cycle vibration days at hours atno. (Ah) test test (V) 80° C. (%) 100° C. (%) 14 1.48 no 0.000 0 4.5 201.48 no 0.001 0 0.2 21 1.48 no 0.001 0 0 22 1.49 no 0.002 0 0 23 1.49 no0.002 0 0 24 1.49 no 0.001 0 0

As shown in Table 12, the battery examples having insulating layers 31on negative electrode 3 show similar results as the battery examplehaving insulating layers 31 on positive electrode 1 shown in Table 6. Inother words, insulating layers 31 made of a heat-resistant material donot shrink even at 100° C.

As described above, Batteries no. 1 to 12 have core-exposed portion 2Cin the positive electrode only, and Batteries no. 13 to 24 havecore-exposed portion 2A in the negative electrode only. The following isa description of battery examples having the core-exposed portions inboth the positive and negative electrodes as shown in FIG. 2A.

In Battery no. 25, electrode assembly 5 is formed using the positiveelectrode of Battery no. 2 and the negative electrode of Battery no. 14.The end of the negative-electrode mixture layer is provided so as toprotrude as long as 2 mm from the end of positive-electrode mixturelayer. Later, current collector 6A is collectively welded to the woundparts of core-exposed portion 2A formed in negative electrode 3, andcurrent collector 6C is collectively welded to the wound parts ofcore-exposed portion 2C formed in positive electrode 1. Electrodeassembly 5 is housed in battery can 7; current collector 6C is connectedto an unillustrated lid; and current collector 6A is connected tobattery can 7. Finally, battery can 7 is filled with the non-aqueouselectrolytic solution and sealed with the lid, thereby completing thenon-aqueous electrolyte secondary battery as Battery no. 25.

Battery no. 26 is prepared in the same manner as Battery no. 25 exceptfor using the positive electrode of Battery no. 3 and the negativeelectrode of Battery no. 14. Battery no. 27 is formed in the same manneras Battery no. 25 except for using the positive electrode of Battery no.2 and the negative electrode of Battery no. 15.

Batteries no. 28 to 31 are prepared in the same manner as Battery no. 25except for using the negative electrode of Battery no. 14 and thepositive electrodes of Batteries no. 4 to 7, respectively. Batteries no.32 to 35 are prepared in the same manner as Battery no. 25 except forusing the positive electrode of Battery no. 2 and the negativeelectrodes of Batteries no. 16 to 19, respectively.

Batteries no. 36 to 40 are prepared in the same manner as Battery no. 25except that separator 4 is not provided and that the same insulatinglayers 31 as in Batteries no. 8 to 12 are provided on positive electrode1. Specifications and test results of the battery examples are shown inTable 13 and Table 14, respectively.

TABLE 13 Positive Negative electrode electrode Mixture Mixture SeparatorWidth density Width density or Battery ratio ratio ratio ratioinsulating no. B/(A + B) (%) B/(A + B) (%) layer 25 0.2 70 0.2 70polyethylene 26 0.3 70 0.2 70 polyethylene 27 0.2 70 0.3 70 polyethylene28 0 105 0.2 70 polyethylene 29 0.2 99 0.2 70 polyethylene 30 0.2 40 0.270 polyethylene 31 0.2 30 0.2 70 polyethylene 32 0.2 70 0 105polyethylene 33 0.2 70 0.2 99 polyethylene 34 0.2 70 0.2 40 polyethylene35 0.2 70 0.2 30 polyethylene 36 0.2 70 0.2 70 aramid resin 37 0.2 700.2 70 alumina 38 0.2 70 0.2 70 titania 39 0.2 70 0.2 70 zirconia 40 0.270 0.2 70 magnesia

TABLE 14 OCV Lithium decrease Shrinkage Shrinkage Discharge depositionin after 3 after 6 Battery capacity in cycle vibration days at hours atno. (Ah) test test (V) 80° C. (%) 100° C. (%) 25 1.48 deposited 0.000 04.6 26 1.25 no 0.000 0 4.6 27 1.49 no 0.000 0 4.7 28 1.48 no 0.000 0 4.229 1.49 no 0.000 0 4.2 30 1.48 no 0.002 0 4.4 31 1.48 no 0.025 0 4.9 321.48 deposited 0.000 0 4.3 33 1.48 no 0.000 0 4.3 34 1.49 no 0.001 0 4.735 1.49 deposited 0.022 0 5.1 36 1.48 no 0.001 0 0.1 37 1.49 no 0.002 00.0 38 1.48 no 0.002 0 0.0 39 1.49 no 0.003 0 0.0 40 1.48 no 0.002 0 0.0

As shown in Table 14, Batteries no. 25 to 40 having the core-exposedportions in both of the positive and negative electrodes cause phenomenasimilar to Batteries no. 1 to 24 having the core-exposed portion ineither the positive or negative electrode, and solve the problems in asimilar manner.

As described hereinbefore, the non-aqueous electrolyte secondary batteryof the present invention can be manufactured at low cost by eliminatingthe process of removing the mixture layers and reducing the number ofexpendable supplies. The battery can also have high safety and highcycle life characteristics by reducing the weight of the positiveelectrode near the end portion of the coated mixture so as to reduce thedesign load of the opposing negative electrode. In addition, thenon-aqueous electrolyte secondary battery and the battery pack canachieve high safety and high output levels by using a heat-resistantresin which reduces separator shrinkage and by disposing heat-resistantporous insulating layers between the electrodes so as to avoid a shortcircuit in case the separator shrinks. The non-aqueous electrolytesecondary battery and battery pack are useful as the power supply forelectric tools, power-assisted bicycles, electric scooters, robots, andthe like.

1. A non-aqueous electrolyte secondary battery comprising: an electrodeassembly including: a first electrode having a first electrode coresheet and first-electrode mixture layers formed by first-electrodemixture including a first active material, each of the first-electrodemixture layers being formed on both sides of the first electrode coresheet; a second electrode having a second electrode core sheet andsecond-electrode mixture layers formed by second-electrode mixtureincluding a second active material, each of the second-electrode mixturelayers being formed on both sides of the second electrode core sheet;and an insulating layer electrically insulating the first electrode andthe second electrode and permeating lithium ions, the electrode assemblybeing formed by winding the first electrode, the second electrode andthe insulating layer together along a winding axis; and a non-aqueouselectrolyte disposed between the first electrode and the secondelectrode, wherein the first electrode includes a first core-exposedportion disposed at an edge of the first electrode core sheet anddisposed perpendicular to the winding axis, the first electrode coresheet being exposed at the first core-exposed portion; and wherein atleast one of the first-electrode mixture layers includes: a firstinclined weight region in which a density of the first-electrode mixturedecreases toward the first core-exposed portion, and a first constantweight region which is adjacent to the first inclined weight region andin which the density of the first-electrode mixture is constant.
 2. Thenon-aqueous electrolyte secondary battery according to claim 1, whereinthe first inclined weight region has a width, in a direction parallel tothe winding axis, of not more than 20% of a width, in a directionparallel to the winding axis, of the first-electrode mixture layers andthe average density of the first-electrode mixture of not less than 40%and not more than 99% of a density of the first-electrode mixture in thefirst constant weight region.
 3. The non-aqueous electrolyte secondarybattery according to claim 1, wherein the second electrode includes asecond core-exposed portion disposed at an edge of the second electrodecore sheet and perpendicular to the winding axis, the second electrodecore sheet exposed at the second core-exposed portion; and wherein atleast one of the second-electrode mixture layers includes: a secondinclined weight region in which a density of the second-electrodemixture decreases toward the second core-exposed portion, and a secondconstant weight region which is adjacent o the second inclined weightregion and in which the density of the second-electrode mixture isconstant.
 4. The non-aqueous electrolyte secondary battery according toclaim 3, wherein the second inclined weight region has a width, in adirection parallel to the winding axis, of not more than 20% of a width,in a direction parallel to the winding axis, of the second-electrodemixture layers and the average density of the second-electrode mixtureof not less than 40% and not more than 99% of a density of thesecond-electrode mixture in the second constant weight region.
 5. Thenon-aqueous electrolyte secondary battery according to claim 1, whereinthe first electrode is a positive electrode and the second electrode isa negative electrode.
 6. The non-aqueous electrolyte secondary batteryaccording to claim 1, wherein the first electrode is a negativeelectrode and the second electrode is a positive electrode.
 7. Thenon-aqueous electrolyte secondary battery according to claim 3, whereinthe first electrode is a positive electrode and the second electrode isa negative electrode.
 8. The non-aqueous electrolyte secondary batteryaccording to claim 1, wherein the insulating layer contains aheat-resistant resin having a heat deflection temperature of not lessthan 200° C.
 9. The non-aqueous electrolyte secondary battery accordingto claim 8, wherein the heat-resistant resin is an aramid resin.
 10. Thenon-aqueous electrolyte secondary battery according to claim 1, whereinthe insulating layer contains insulating filler.
 11. The non-aqueouselectrolyte secondary battery according to claim 10, wherein theinsulating filler is an inorganic oxide.
 12. The non-aqueous electrolytesecondary battery according to claim 1, wherein the insulating layer isone of layers formed on at least ones of the first-electrode mixturelayers and the second-electrode mixture layers.
 13. The non-aqueouselectrolyte secondary battery according to claim 1, further comprising aseparator disposed between the first electrode and the second electrode.14. The non-aqueous electrolyte secondary battery according to claim 13,wherein the insulating layer is formed on the separator.
 15. A batterypack comprising: a non-aqueous electrolyte secondary battery; and anouter case for covering the non-aqueous electrolyte secondary battery,wherein the non-aqueous electrolyte secondary battery having: anelectrode assembly including: a first electrode having a first electrodecore sheet and first-electrode mixture layers formed by first-electrodemixture including a first active material, each of the first-electrodemixture layers being formed on both sides of the first electrode coresheet; a second electrode having a second electrode core sheet andsecond-electrode mixture layers formed by second-electrode mixtureincluding a second active material, each of the second-electrode mixturelayers being formed on both sides of the second electrode core sheet;and an insulating layer electrically insulating the first electrode andthe second electrode and permeating lithium ions, the electrode assemblybeing formed by winding the first electrode, the second electrode andthe insulating layer together along a winding axis; and a non-aqueouselectrolyte disposed between the first electrode and the secondelectrode, wherein the first electrode includes a first core-exposedportion disposed at an edge of the first electrode core sheet anddisposed perpendicular to the winding axis, the first electrode coresheet being exposed at the first core-exposed portion; and wherein atleast one of the first-electrode mixture layers includes: a firstinclined weight region in which a density of the first-electrode mixturedecreases toward the first core-exposed portion, and a first constantweight region which is adjacent to the first inclined weight region andin which the density of the first-electrode mixture is constant.